The present invention relates to a laser recorder using a semiconductor laser which is capable of reproducing an image such as a picture having half-tones with a high quality.
In intensity-modulating a laser beam for recording an image having half-tones (hereinafter referred to as "a half-tone image" when applicable), any of (1) a technique of using an ultrasonic optical modulator, (2) a technique of varying the discharge current of a gas laser, and (3) technique of varying the current of a semiconductor laser may be employed.
The first technique is disadvantageous in that it is expensive and requires an intricate construction because of the need for an expensive ultrasonic optical modulator and a fine adjustment mechanism for matching the Bragg angles of the modulator.
The second technique is also disadvantageous in that the modulating frequency is in a low frequency band of the order of several hundreds of Hertz and the service life of the laser tube is reduced by varying the discharge current.
The third technique suffers from the drawback that since the optical output vs. current characteristic curve of the semiconductor laser is as shown in FIG. 1, the optical output is greatly varied merely by slightly changing the input current. Therefore it is considerably difficult to record a half-tone image by controlling the optical output in an analog mode by varying the input. However, the semiconductor laser is advantageous in that it can be subjected to binary modulation with a high frequency signal and therefore it can be used for optical communication.
A method wherein an input signal is sampled with a sampling pulse, and a high frequency pulse signal having a frequency of at least 10 Hz is produced using the sampling pulse so that the number of high frequency pulses outputted in each sampling period is controlled according to a semiconductor laser thereby to record a half-tone image (hereinafter referred to as "a pulse number modulation method" when applicable) and a method wherein a pulse width modulation signal having a pulse width corresponding to the number of high frequency pulses is applied to a semiconductor laser to record a half-tone image (hereinafter referred to as "a pulse width modulation method" when applicable) have been disclosed In U.S. patent application Ser. No. 214,815 filed Dec. 9, 1980 (corresponding to Japanese patent application No. 168565/1979) filed by the present applicant.
An object of the invention is to provide a laser recorder which can reproduce a half-tone image with high accuracy by the utilization of the binary modulation capability of the semiconductor laser.
The term "sampling pulse" as herein used is intended to mean a pulse for sampling an input video signal at predetermined time intervals. The frequency of the sampling pulse can be selected as desired. However, it is preferable that it be slightly higher than the maximum frequency of the video signal in order to reproduce the image with a high resolution. Furthermore, the term "high frequency pulse" is intended to mean a pulse having a frequency higher than that of the sampling pulse mentioned above. Preferably, the frequency of the high frequency pulse is several hundred to several thousand times that of the sampling pulse. There two pulses may be generated separately although it is preferable that the sampling pulse be obtained by subjecting the high frequency pulse to frequency division.
The amount of exposure of each of the picture elememts which form an image is determined by the pulse width T of a pulse width modulation signal which is applied to a semiconductor laser according to the level of an input video signal during the respective sampling period. In accordance with the invention, the pulse width T corresponds to the number N of high frequency pulses which are outputted during each sampling period. If the pulse width increase required whenever the number of high frequency pulse is increased by one in order to maintain the total amount of disclosure constant (hereinafter referred to as "a unitary pulse" when applicable) is represented by .DELTA.t, then the corresponding pulse width T is: EQU T=N.multidot..DELTA.t (1)
If, when the pulse width modulation signal is at a high "H" logic level, i.e. when the light beam is being applied to a photosensitive material, the light intensity is represented by I, then the total optical energy applied to a picture element, i.e., the exposure E, is defined by the following expression: ##EQU1## In expression (2), the light intensity I and the unitary pulse width .DELTA.t are constant, and therefore the exposure E is proportional to the number N of high frequency pulses (hereinafter referred to as "a high-frequency pulse number N" when applicable). If the increment of exposure per high frequency pulse (hereinafter referred to as "a unitary exposure" when applicable) is represented by .DELTA.e, then: EQU .DELTA.e=I.multidot..DELTA.t (3)
Using, expression (3), expression (2) can be written as the following expression (4): EQU E=N.multidot..DELTA.e (4)
The above description will become more apparent when considered along with FIG. 2.
Next, the relation between high-frequency pulse numbers N and densities of an image recorded in the case where the image is recorded by a semiconductor laser using a pulse width modulation signal having a pulse width corresponding to a high-frequency pulse number will be described with reference to FIG. 3.
In FIG. 3, a curve I is an example of the characteristic curve of a recording material indicating the logarithm of the exposure amounts E with density, and a curve II is an example of the relation between the numbers N of high frequency pulses and the logarithms of exposure amounts E of the recording material which are determined from expression (4).
In FIG. 3, once a density level has been selected, the corresponding high-frequency pulse number N can be obtained by following the arrow. When the density level D is changed from 0.1 to 0.2 in the low density range, the high-frequency pulse number N increases by only about nine. However, when the density level D is changed from 1.3 to 1.4 in the high density range, the high-frequency pulse number N increases by about fifty pulses.
As is apparent from the above description, in order to reproduce the gradations of an image with a sufficiently high accuracy at equal density intervals, the frequency of the high frequency pulse must be much higher than that of the sampling pulse, for instance, several hundred times or, if necessary, several thousand times.
The relation of the sampling pulse frequency f.sub.s, the high-frequency pulse frequency f.sub.H, and the maximum level of the input signal, i.e., the maximum pulse number N.sub.max which is required for the level of the input signal to which the maximum exposure corresponds is: EQU f.sub.H &gt;.gtoreq.N.sub.max .times.f.sub.s ( 5)
The maximum pulse number N.sub.max will be larger than the values specified in FIG. 3 if the density intervals are sufficiently small to reproduce the image with a high accuracy or for certain ranges of the characteristic of the photosensitive material such as .gamma. (the maximum gradient of the characteristic curve) and a range of density D. As a result, the high-frequency pulse frequency is greatly increased making it difficult to provide circuitry implementing the above-described modulation method.
By way of examples, if the sampling frequency f.sub.s =100 KHz, and the maximum pulse number N.sub.max =500, the corresponding necessary high-frequency pulse frequency f.sub.H is: EQU f.sub.H .gtoreq.N.sub.max .times.f.sub.s =50 MHz.
Accordingly, a circuit for practicing the above-described modulation method cannot be constructed of standard TTL (transistor-transistor-logic) elements. Thus, the conventional modulation method is disadvantageous in that ECL (emitter-coupled logic) elements or the like must be used to implement the circuit and hence the circuit has a considerably high manufacturing cost.